Heat-resistant synthetic insulating paper called as “aramid paper” has recently been provided as an electrical insulating material. More specifically, for example, aromatic polyamide paper is known under the trademark of Nomex (registered trademark) owned by E.I. du Pont de Nemours and Company, U.S.A. (hereinafter, DuPont), such as Nomex #410 and #411. This aromatic polyamide paper has excellent heat resistance, high mechanical properties, and satisfactory electrical insulation and has a thickness ranging from 2 to 20 milli-inch (mil). This aramid paper is used as an insulating material for transformers, electric motors and electric generators all of which are needed to have high heat resistance belonging to type H (180° C.) in a heat resistance division of International Electrotechnical Commission (IEC) 85 (1984).
The aramid paper is made mainly from pulp-like aramid (aromatic polyamide) fibrids and heat-resistant aramid short fibers by a wet papermaking process similar to a method of making Japanese paper as WASHI. If necessary, the aramid paper is heated and pressurized (calendered) to be commercially available. The Nomex® #410 is a calendered product, whereas the Nomex® #411 is a non-calendered product. Each of these aramid papers contains a number of voids therein as normal paper does. Accordingly, the aramid papers necessarily have a lower breakdown voltage (BDV: in KV/mm) per thickness than that of a film having the equivalent quality and same thickness.
In contrast, a film of polyester (hereinafter, referred to as “PET film”) such as polyethylene terephthalate or polyethylene naphthalate has a lower resistance to heat than the aramid paper and is classified into a type E (120° C.) in the heat resistance division of IEC 85 (1984). However, the PET film has a higher breakdown voltage and is more inexpensive. The PET film is thereby used in a wide range for the purpose of insulation corresponding to the type E or lower types.
Incidentally, insulating materials belonging to the type F (155° C.) have recently been desired in a market. The insulating materials of the type F do not require so high heat resistance equivalent to those of type H. Aramid papers have no problem in their heat resistance if used in type F but they are expensive. As a result, development of more inexpensive materials has been desired. The following materials (a) to (e) have been proposed as cost-effective type F materials:
(a) A multilayer structure of an aramid paper and a PET film together through an adhesive, in which the aramid paper has high resistance to heat and to oxidation, and the PET film has high electrical insulation;
(b) An aramid laminate prepared by layering an aramid paper and a PET film on each other and pressurizing and heating the laid aramid paper and PET film under the conditions of temperature ranging from 220° C. to 250° C. and linear pressure of 50 kg/cm or more so that the laid aramid paper and PET film are laminated by thermal bonding to give a laminate (see, for example, Patent Document 1: JP-A No. H07-32549 (1995)) . The used aramid paper is m-aramid paper, whereas the used PET film is a biaxially oriented PET film;
(c) A laminate including in order an aramid paper layer (layer A), a PET layer, and a PET film layer, wherein the PET layer is formed by heat melt bonding to or impregnating into a surface of the layer A at a temperature equal to or higher than the melting point of PET. The laminate is prepared by the following processes: layering the PET layer and the PET film on the surface of the layer A in the order these terms appeared, subsequently melt bonding between the PET layer and the PET film at a roll temperature of 220° C. to 250° C. and a pressure of 50 kg/cm or more, and rapidly cooling them at a rate of 100° C./minute or more (see, for example, Patent Document 2: JP-A No. H07-299891 (1995);
(d) A laminate prepared by layering an m-aramid paper having a specific gas permeability and a polyester film having an intrinsic viscosity of 0.6 or more, calendering the m-aramid paper and the polyester film with two calendering rolls maintained at elevated temperatures which differ from each other, specifically a surface of the m-aramid paper layer side roll has a higher roll temperature and a surface of the polyester film layer side roll has a lower roll temperature, after calendaring, and cooling rapidly the two layers at temperature equal to or lower than the glass transition point of the polyester; thereby, the laminate being formed so that the two layers firmly bonded to each other by impregnating only a surface layer of the m-aramid paper layer between the two layers with a molten polyester (see, for example, Patent Document 3: JP-A No. H08-99389 (1996)); and
(e) A prepreg prepared by subjecting a nonwoven fabric made from aramid fibers to a plasma treatment under normal atmospheric pressure, impregnating the treated nonwoven fabric with a thermosetting resin, and heating the impregnated nonwoven fabric to be in a “B-stage” (see, for example, Patent Document 4: JP-A No. H11-209484 (1999)).
The multilayer structure of (a) in which the aramid paper and the PET film are bonded to each other through the adhesive is poor in workability such as bending workability, because the adhesive is relatively hard and this impairs excellent elasticity originated from the material aramid paper and PET film. Furthermore, when the multilayer structure is applied to oil-immersed equipment, components of the adhesive may migrate into the oil, whereby the usage of the multilayer structure is limited. The prepreg of (e) is prepared by impregnating the nonwoven fabric made from aramid fibers with a thermosetting resin. This prepreg also uses the thermosetting resin as an adhesive.
On the other hand, in the aramid laminate of (b) and the laminate of (c), the aramid paper and the PET film are bonded together by heat melt bonding without use of an adhesive, so that the defect resulting from use of adhesive can be overcome. In the aramid laminate of (b), however, since the temperature of heat melt bonding is approximate to a melting point of PET (about 260° C.), variations in the dimensions of PET film are increased. This results in warpage, shrinkage or crease in the bonded products. Furthermore, the PET tends to be crystallized partially. As a result, it is difficult to give products with stable quality. Since the temperature of heat melt bonding is high in the above laminate of (c), too, part of PET impregnated in the aramid paper is crystallized and thereby damage the excellent elasticity.
In the laminate of (d), the polyester is melted, and the aramid fibers or aramid paper is impregnated with the molten polyester. In order to melt the polyester, the polyester must be heated at a temperature higher than its melting temperature. Accordingly, the polyester re-crystallizes upon cooling to decrease its elasticity.
Under these circumstances, an object of the present invention is to provide an aramid paper, and a manufacturing method thereof, which aramid paper is thermally bondable with a nonhydrolyzable resin film such as a polyphenylene sulfide (hereinafter briefly referred to as “PPS”) film at a relatively low temperature without using an adhesive and which has high heat resistance. Another object of the present invention is to provide a laminate of an aramid paper and a resin film, the laminate which has both high heat resistance derived from the constituent aramid paper and satisfactory electrical insulation derived from the constituent nonhydrolyzable resin film and which excels in elasticity.